High Efficiency Video Coding (HEVC) is a new coding standard that has been developed in recent years. In the High Efficiency Video Coding (HEVC) system, the fixed-size macroblock of H.264/AVC is replaced by a flexible block, named coding unit (CU). Pixels in the CU share the same coding parameters to improve coding efficiency. A CU may begin with a largest CU (LCU), which is also referred as coded tree unit (CTU) in HEVC. In addition to the concept of coding unit, the concept of prediction unit (PU) is also introduced in HEVC. Once the splitting of CU hierarchical tree is done, each leaf CU is further split into one or more prediction units (PUs) according to prediction type and PU partition.
Along with the High Efficiency Video Coding (HEVC) standard development, the development of extensions of HEVC has also started. The HEVC extensions include range extensions (RExt) which target at non-4:2:0 color formats, such as 4:2:2 and 4:4:4, and higher bit-depths video such as 12, 14 and 16 bits per sample. One of the likely applications utilizing RExt is screen sharing, over wired- or wireless-connection. Due to specific characteristics of screen contents, coding tools have been developed and demonstrate significant gains in coding efficiency. Among them, the palette coding (a.k.a. major color based coding) techniques represent block of pixels using indices to the palette (major colors), and encode the palette and the indices by exploiting spatial redundancy. While the total number of possible color combinations is huge, the number of colors in an area of picture is usually very limited for typical screen contents. Therefore, the palette coding becomes very effective for screen content materials.
During the early development of HEVC range extensions (RExt), several proposals have been disclosed to address palette-based coding. For example, a palette prediction and sharing technique is disclosed in JCTVC-N0247 (Guo et al., “RCE3: Results of Test 3.1 on Palette Mode for Screen Content Coding”, Joint Collaborative Team on Video Coding (JCT-VC) of ITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG 11, 14th Meeting: Vienna, AT, 25 Jul.-2 Aug. 2013 Document: JCTVC-NO247). In JCTVC-NO247, the palette of each color component is constructed and transmitted. The palette can be predicted (or shared) from its left neighboring CU to reduce the bitrate.
An improved palette prediction and sharing technique is disclosed in JCTVC-O0218 (Guo et al., “Evaluation of Palette Mode Coding on HM-12.0+RExt-4.1”, Joint Collaborative Team on Video Coding (JCT-VC) of ITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG 11, 15th Meeting: Geneva, CH, 23 Oct.-1 Nov. 2013, Document: JCTVC-O0218). In JCTVC-O0218, the encoding process is shown as follows.    1. Transmission of the palette: the palette size (number of colors in the palette) is first transmitted, followed by the palette elements (the color values).    2. Transmission of pixel palette index values (indices pointing to the colors in the palette): the index values for the pixels in the CU are encoded in a raster scan order. For each position, a flag is first transmitted to indicate whether the “run mode” or “copy above mode” is being used.            2.1 “Run mode”: In “run mode”, a palette index is first signaled followed by “palette run” (e.g., M). No further information needs to be transmitted for the current position and the following M positions as they have the same palette index as signaled. The palette index (e.g., i) is shared by all three color components, which means that the reconstructed pixel values are (Y, U, V)=(paletteY[i], paletteU[i], paletteV[i]) (assuming the color space is YUV)        2.2 “Copy above mode”: In “copy above mode”, a value “copy run” (e.g., N) is transmitted to indicate that for the following N positions (including the current one), the palette indices are equal to the palette indices of the ones that are at the same positions in the row above.            3. Transmission of residue: the palette indices transmitted in Stage 2 are converted back to color values and used as the predictor. Residue information is transmitted using HEVC residue coding and is added to the prediction for the reconstruction.
Another palette coding technique is disclosed in JCTVC-O-0182 (Guo et al., “AHG8. Major-color-based screen content coding”, Joint Collaborative Team on Video Coding (JCT-VC) of ITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG 11, 15th Meeting: Geneva, CH, 23 Oct.-1 Nov. 2013, Document: JCTVC-O0182). However, instead of predicting the entire palette from the left CU, individual palette color entry in a palette can be predicted from the exact corresponding palette color entry in the above CU or left CU. In other words, JCTVC-O-0182 discloses an element-by-element palette prediction. Three types of line modes are used for predicting each index line, i.e. horizontal mode, vertical mode and normal mode. In the horizontal mode, all the indices in the same line have the same value. If the value is the same as the first pixel of the above pixel line, only the line mode signaling bits are transmitted. Otherwise, the index value is also transmitted. In vertical mode, the current index line is the same with the above index line. Therefore, only line mode signaling bits are transmitted. In normal mode, indices in a line are predicted individually. For each index position, the index of the left or above neighboring sample is used as predictor, and the prediction symbol is transmitted to the decoder.
Furthermore, JCTVC-O-0182 discloses a technique that classifies pixels into major color pixels (with palette indices pointing to the palette colors) and escape pixel. For major color pixels, the decoder reconstructs pixel value according to major color index (also referred as palette index) and palette. For escape pixel, the encoder would further send the pixel value.
Therefore, it is desirable to develop methods for further improving the coding efficiency and/or reducing the complexity associated with the palette coding.